1. Field of the Invention
This invention relates to an osteoimplant of predetermined dimensions and shape made up of a coherent aggregate of elongate bone particles and to a method for making the osteoimplant. Among its other applications, the osteoimplant can be fashioned as a plug for insertion in a space or cavity within an implant used in an orthopedic procedure, e.g., an intervertebral spacer employed in spinal fusion, or for insertion in a cavity associated with a relatively well-defined bone defect, e.g., an extraction socket, a bore hole, etc.
2. Description of the Related Art
Shaped or cut bone elements have been used extensively to treat various medical problems in human and animal orthopedic surgical practice. The use of such bone has also extended to the fields of, e.g., cosmetic and reconstructive surgery, dental reconstructive surgery, podiatry, orthopaedics, neurosurgery and other medical fields involving hard tissue. The use of autograft bone (where the patient provides the source), allograft bone (where another individual of the same species provides the source) or xenograft bone (where another individual of a different species provides the source) is well known in both human and veterinary medicine. In particular, transplanted bone is known to provide support, promote healing, fill bony cavities, separate bony elements (such as vertebral bodies), promote fusion (where bones are induced to grow together into a single, solid unit) or stabilize the sites of fractures. More recently, processed bone has been developed into shapes for use in new surgical applications or as new materials for implants that were historically based on non-biologically derived materials.
A particularly advantageous application of shaped or cut bone elements, particularly those derived from allograft bone, is that involving the fusion of adjacent vertebral bodies where there has been damage or injury to the intervertebral disc.
Intervertebral discs, located between the end plates of adjacent vertebrae, stabilize the spine, distribute forces between vertebrae and cushion vertebral bodies. A normal intervertebral disc includes a semi-gelatinous component, the nucleus pulpous, which is surrounded and confined by an outer fibrous ring, the annulus fibrous. In a healthy, spine, the annulus fibrous prevents the nucleus pulpous from protruding outside the disc space.
Spinal discs may be displaced or damaged due to trauma, disease, or aging. Disruption of the annulus fibrosis allows the nucleus pulposus to protrude into the vertebral canal, a condition commonly referred to as a herniated or ruptured disc. The extruded nucleus pulposus may press upon a spinal nerve, resulting in nerve damage, pain, numbness, muscle weakness and/or paralysis. Intervertebral discs may also deteriorate due to the normal aging process or disease. As a disc dehydrates and hardens, the disc space height will be reduced leading to instability of the spine, decreased mobility and pain.
Sometimes the only relief from the symptoms of these conditions is a discectomy or surgical removal of a portion or all of an intervertebral disc followed by fusion of the adjacent vertebrae. The removal of the damaged or unhealthy disc will allow the disc space to collapse. Collapse of the disc space can cause instability of the spine, abnormal joint mechanics, premature development of arthritis or nerve damage in addition to severe pain. Pain relief via discectomy and arthrodesis requires preservation of the disc space and eventual fusion of the affected motion segments.
Bone grafts have been used to fill the intervertebral space to prevent disc space collapse and promote fusion of adjacent vertebrae across the disc space. In early techniques, bone material was simply implanted between adjacent vertebrae, typically at the posterior aspect of the vertebra, and the spinal column was stabilized by way of a plate or rod spanning the affected vertebrae. Once fusion occurred, the hardware used to maintain the stability of the vertebrae became superfluous and became a permanent non-functional foreign body. Moreover, the surgical procedures employed to implant a rod or plate to stabilize the spine while fusion was taking place were frequently lengthy and involved.
It was therefore determined that a better solution to the stabilization of an excised disc space is to fuse the vertebrae between their respective end plates, preferably without the need for anterior or posterior rod or plate. There have been numerous attempts to develop an acceptable intradiscal implant that could be used to replace a damaged disc and maintain the stability of the disc inter space between the adjacent vertebrae, at least until complete arthrodesis is achieved. The implant must provide temporary support and allow bone ingrowth. Success of the discectomy and fusion procedure requires the development of a contiguous growth of bone to create a solid mass because the implant may not withstand the compressive loads on the spine for the life of the patient.
Fusion cages provide a space for inserting a bone graft between adjacent portions of bone. In time, the bone and bone graft grow together through or around the fusion cage to fuse the graft and the bone solidly together. One current use of fusion cages is to treat a variety of spinal disorders, including degenerative disc diseases such as Grade I or II spondylolisthesis of the lumbar spine. Spinal fusion cages (included in the general term, “fusion cages”) are inserted into the intervertebral disc space between two vertebrae for fusing them together. They distract (or expand) a collapsed disc space between two vertebrae to stabilize the vertebrae by preventing them from moving relative to each other.
The typical fusion cage is cylindrical, hollow and threaded. Alternatively, some known fusion cages are unthreaded or made in tapered, elliptical, or rectangular shapes. Known fusion cages are constructed from a variety of materials including titanium alloys, porous tantalum, other metals, allograft bone, or ceramic material. For example, U.S. Pat. No. 5,015,247 to Michelson and U.S. Pat. No. 5,782,919 to Zdeblick disclose a threaded spinal cage the contents of which are incorporated herein by reference. The cages are hollow and can be filled with osteogenic material, such as autograft or allograft, prior to insertion into the intervertebral space. Apertures defined in the cage communicate with the hollow interior to provide a path for tissue growth between the vertebral end plates.
Fusion cages may be used to connect any adjacent portions of bone. A primary use is in the lumbar spine. Other sites include the cervical or thoracic segments of the spine. Fusion cages can be inserted in the lumbar spine using an anterior, posterior or lateral approach. Insertion is usually accomplished through a traditional open operation but a laparoscopic or percutaneous insertion technique can also be used.
Spinal fusion cages are typically designed to support vertebrae in the proper geometry during the fusion process and are not intended to provide a long term, permanent support. Actual bone fusion is the ultimate goal. In order to achieve fusion, bone conducting or inducing materials such as bone chips, ceramics, marrow, growth factors, etc., are packed into the cage in order to provide a favorable environment for bony ingrowth. The success of these methods depends to some extent on the surgeon's skill in packing the cages and retaining the materials in the cages during and after implantation. It is especially important that the filler material be in contact with the surfaces of the vertebral bodies on either side of the cage as well as any autograft material(s) employed at the surgical site. While many materials will function as adequate cage filler materials, the best results are obtained with materials that are osteoinductive and not just osteoconductive. Bone grafting materials for use in osteoimplants are described in U.S. Pat. No. 4,950,296, the contents of which are incorporated by reference herein.
Osteoconductive materials are ones that guide bone growth but do not stimulate it. Examples are bone chips and ceramics. Osteoinductive materials actually cause bone to form and result in faster and more certain healing. Examples of osteoinductive materials include cancellous bone, demineralized bone and various growth factors. The most common source of osteoinductive material is the patient's own bone. Typically, in spinal surgery, this is harvested from the iliac crest in the form of bone chips and marrow. While effective, it causes secondary damage (to the harvest site) and requires preparation before it can be used. Furthermore, it is somewhat difficult to maintain in place due to its semi-fluid nature.
Demineralized bone is an alternative to bone chips and marrow as an osteoinductive material. Demineralized bone comes in various forms including powder, gels, pastes, fibers and sheets. The more fluid forms such as powders, gels and pastes are relatively easy to implant at the repair site but difficult to maintain in place. The production of bone powder for filling osteoimplants is disclosed and incorporated herein by reference to U.S. Pat. No. 5,910,315 et al. The process of preparing shaped materials derived from elongate bone particles is incorporated herein by reference to U.S. Pat. No. 5,507,813.
In addition, there is the possibility of wasted material anytime a standard material has to be adapted to fill a cage. Therefore, the need remains for an osteoinductive material that can be used to fill the relatively well-defined cavities of, for example, bone fusion devices, extraction sockets, bore holes, etc. and that doesn't require any special tailoring by the surgeon at the time of implantation yet remains where placed for periods of time sufficient to allow suitable bone fusion to take place. It would be advantageous if methods of producing such a material could be achieved efficiently and accurately by a simple process. The use of such an osteoinductive material at an appropriate surgical site would provide improved outcome for implant recipients.
U.S. Pat. No. 5,507,813 describes a surgically implantable sheet formed from elongate bone particles, optionally those that have been demineralized. The sheet can further contain biocompatible ingredients, adhesives, fillers, plasticizers, etc. The osteoinductive sheet is rigid and relatively strong when dry and flexible and pliable when wetted or hydrated. These sheets are available under the tradename Graflon® Flex (Osteotech, Inc., Eatontown, N.J., USA). The sheets must be wetted/hydrated prior to use in order to render them useful for implantation.
U.S. Pat. No. 4,932,973 describes an artificial organic bone matrix with holes or perforations extending into the organic bone matrix. The holes or perforations are indicated to be centers of cartilage and bone induction following implantation of the bone matrix into living tissue.
U.S. Pat. No. 4,394,370 discloses a one-piece sponge-like bone graft material fabricated from fully demineralized bone powder or microparticulate bone, and reconstituted collagen. The sponge-like graft is optionally crosslinked with glutaraldehyde.
Another one-piece porous implant is described in U.S. Pat. No. 5,683,459. The implant is made up of a biodegradable polymeric macrostructure composed of chemotactic ground substances such as hyaluronic acid.